1. Field of the Invention
The present invention relates to a printed circuit board (PCB), and more particularly, to a printed circuit board comprising an embedded internal functional element.
2. Description of the Prior Art
Since the portable electronic products (such as cellular phone, notebook computer, hand-held camera and personal digital assistant, etc.) are getting more and more popular nowadays, over-current protection apparatuses for avoiding the occurrence of over-current or the over-temperature of the portable electric products are increasingly important.
In the protection apparatuses, a positive temperature coefficient (PTC) over-current protection apparatus is used extensively because of its characteristics of being resettable, sensitive to temperature and stable in reliability. Thus, the PTC over-current protection apparatus has been widely applied to protect batteries, especially second batteries, such as the nickel-hydrogen battery or the lithium battery, etc.
A PTC conductive composition material (PTC material) is utilized as a current sensitive element of the PTC over-current protection apparatus, because the resistance of the PTC material is very sensitive to temperature variation. Because the resistance of the PTC material is very low at its normal temperature, the circuit can operate normally. However, if the over-current or over-temperature of the battery occurs because of improper usage, the resistance of the PTC material will increase immediately for at least ten thousand times (such as 104 ohm) so that the PTC material will be at a high-resistance state. Therefore, the over current will be counterchecked and thereby the object of protecting circuit elements of the battery is achieved.
FIG. 1 is a schematic diagram showing connection of a conventional PTC over-current protection apparatus. One terminal of the PTC over-current protection apparatus 12, mounted on the surface of a printed circuit board 10, is connected to a power supply 11 and the other terminal is connected to a first integrated circuit 13. Generally, the normal value of the electrical resistance of the PTC over-current protection apparatus 12 is obtained according to the conventional formula:       R    =                  ρ        xc3x97        l            A        ,
in which R is the resistance in ohms, xcfx81 is the resistivity in ohms-cm of the PTC material, l is the length between two electrodes and A is the effective area of the protection apparatus 12. Since the size of the printed circuit board of the portable electronic product becomes smaller and smaller, the footprint of the PTC over-current protection apparatus 12 mounted on the printed circuit board also needs to be decreased comparatively. According to the above formula, as the normal resistance of the PTC over-current protection apparatus 12 is increased, the power consumption will increase such that the working voltage of the first integrated circuit 13 connected with the protection apparatus will be dropped.
Furthermore, the printed circuit board has the trend of small size and high density at the present day. Therefore, the number of internal layers of the PCB has increased to be even more than 12, especially in small, thin and light products, such as cellular phone, personal digital assistant (PDA) and digital camera, etc. Generally, a main process of processes for manufacturing the multi-layer printed circuit board is a so-called build-up process. The build-up process forms the printed circuit board by stacking a circuit layer and an insulation layer one by one, such that a multi-layer PCB having vias and high density is formed.
FIGS. 2(a) to 2(e) depicts a conventional build-up process. In FIG. 2(a), a substrate 20 is provided, which is composed of a glass fiber and a resin. A first conductive layer 21, such as a copper foil, is applied to the surface of the substrate 20. In FIG. 2(b), the first conductive layer 21 is etched by a chemical method for forming an isolating area 22. In FIG. 2(c), an insulating layer 23 is applied to the surface of the first conductive layer 21. In FIG. 2(d), the insulating layer 23 is etched by a laser or chemical method for forming a conductive via 24. In FIG. 2(e), a second conductive layer 25 is applied to the insulating layer 23 by a method of plating or electroless plating. In the above-mentioned plating process, the conductive via 24 will be filled with a conductive material to conduct the first conductive layer with the second conductive layer. Therefore, the conductive layer and the insulating layer can be stacked one by one if the above steps are repeated over and over, and thus a multi-layer printed circuit board is formed. Moreover, any two conductive layers of the printed circuit board can also be connected by a electrically conductive hole. The electrically conductive hole is formed by mechanical drilling and plating the hole, electrolessly plating the hole or filling the hole with a conductive paste so as to connect two conductive layers together. FIG. 3 depicts a diagram of the electrically conductive hole; wherein a first conductive layer 31 is connected to a second conductive layer 32 and a second terminal point 36 through a first conductive hole 33. However, if the first conductive layer 31 needs to be connected to a first terminal point 35 without being connected to a second conductive layer 32, an etched area 37 is formed around a second conductive hole 34 in the second conductive layer 32, and thus the second conductive hole 34 and the second conductive layer 32 are isolated.
Since the size of the printed circuit board 10 is decreased, the area for mounting the electrical apparatus is limited. Therefore, how to increase the utilization area of the printed circuit board is a critical problem to be tackled with. For this purpose, the present invention discloses a printed circuit board having an embedded internal over-current protection apparatus to increase the utilization area of the PCB and decrease the normal resistance. Moreover, the surface of the PCB can accommodate more devices, since the over-current protection apparatus mounted on the PCB surface is not necessary. On the other hand, an external damage to the surface mount over-current protection apparatus will be avoided.
A major object of the present invention is to provide a printed circuit board (PCB) with an increased area of an embedded internal functional element for the benefit of decreasing the normal resistance. Thus, the power consumption of this invention is much lower than an over-current protection apparatus being surface mounted on the PCB surface and the dropping of the working voltage will also be significantly reduced.
A second object of the present invention is to make the embedded functional element constituted by one or more than one internal layers of the printed circuit board. Since this embedded functional element design utilizes more effective area of over-current protection apparatus without utilizing any area of PCB surface, the resulted electrical rating of embedded functional element in the printed circuit board such as maximum working current is higher.
A third object of the present invention is to make the functional element to constitute a resistive or sensing element, and thus the number of the apparatuses mounted on the surface of the PCB is decreased and the utilization rate of the PCB is improved.
In order to achieve the above objects and to avoid the disadvantages of the prior art, the present invention discloses a PCB, characterized in that besides the conductive layer and the insulating layer, the PCB further comprises at least one functional element either current-sensitive or temperature-sensitive layer, such as the positive temperature coefficient (PTC) element,negative temperature coefficient (NTC) element, or zero temperature coefficient (ZTC) element. The functional element comprises a functional material, an upper electrode and a lower electrode, and the functional material is selected from the group consisting of PTC material, ZTC material and NTC material. If the PTC element is applied in the present invention, the normal resistance of the present invention will be smaller than that of conventional PTC protection apparatus since the area of the PTC element of the present invention is larger than that of conventional PTC protection apparatus. Moreover, through an electrically conductive hole, an upper electrode and a lower electrode respectively lying on the top and bottom surfaces of the functional PTC element are respectively connected with an apparatus mounted on the surface of the PCB to form a connecting circuit. Thus, the PTC overcurrent protection apparatus which is usually mounted on the surface of the PCB is eliminated, and the surface utilization rate of the PCB will be improved while the over-current protection mechanism through the entire circuitry is still maintained.
The foregoing and other objects and advantages of the invention and the manner in which the same are accomplished will be clearly shown based on the following detailed description taken in conjunction with the accompanying drawings.